This invention relates generally to cable testing and troubleshooting, and in particular to measuring the crosstalk characteristics between adjacent pairs of cables and extracting diagnostic information to locate faults in cables.
Local Area Networks (LANs) formed by a large number of interconnected computers, work stations, printers, and file servers are becoming increasingly common in the modern office. In a typical installation, LAN cables may be routed through walls, floors, and ceilings of a building. LAN cable systems not only need to be checked out upon installation, but they require constant maintenance, upgrades, and troubleshooting because LAN cables and connectors are subject to breakage, offices and equipment must be moved, and new equipment are added. ALAN system is most commonly implemented by physically connecting all of these devices with copper-conductor, twisted-pair LAN cables, the most common being an eight-wire cable which is configured in four twisted-wire pairs (commonly referred to simply as "twisted pairs") within a flexible wrapper that may include an electrostatic shield, with each end of the cable terminated in an industry-standard connector.
Crosstalk level is a performance parameter indicating the level of signal coupling between separate but adjacent data transmission paths, and thus is of great importance for determining problems in twisted-pair cables in LAN systems. It is desirable that the level of isolation be high (and crosstalk level be low) to avoid interference between transmission paths. Accordingly, crosstalk measurement capability is highly desirable in any test tool to be used in LAN cable testing and troubleshooting.
More sophisticated LAN cable test instruments are often equipped for evaluating crosstalk through standardized near-end crosstalk (commonly referred to as "NEXT") measurements. A conventional method of measuring crosstalk, or NEXT, is to apply a high-frequency sine wave signal to one twisted pair of wires in the cable and measuring the crosstalk signal level in another of the twisted pairs of wires. The sine wave source is incremented in discrete steps across a range of frequencies, while making measurements, and a list of crosstalk versus frequency is generated. This list is compared with a worst-case crosstalk versus frequency function specified for the cable installation under test to determine compliance-that is, whether or not crosstalk levels are acceptable or not. However, this conventional method does not yield any useful diagnostic information to inform the user just exactly what the problem is nor how to fix it. For example, unacceptable crosstalk may be a result of a fault in the cable, such as the cable being physically crushed at some point, or simply a poor quality patch cord somewhere in a LAN run.
Conventional time domain reflectometers (TDRs) locate faults caused by substantial changes in impedance in a cable, such as opens and shorts, by measuring the elapsed time between a stimulus pulse and a reflected pulse on the same cable; however, such conventional TDRs cannot provide a crosstalk versus frequency function, nor locate a problem such as a faulty or poor quality patch cord that has a perfectly good impedance match to the twisted-pair LAN cable.